Ultra-Large-Coupling and Spurious-Free SH0 Plate Acoustic Wave Resonators Based on Thin LiNbO3.

This study lays a foundation for the design of the SH0 plate acoustic wave (PAW) resonators based on LiNbO3 plate, and are applicable to the ultra-wideband filters and next-generation tunable filters. The coupling coefficient ( k2 ) of SH0 mode is optimized to as high as 55% and wideband spurious are well controlled by analyzing the propagation characteristics of plate modes in LiNbO3. The SH0 mode is demonstrated to be slowly dispersive and features the superiority of interdigital transducer (IDT)-defining frequency over most other plate modes. On the device level, the transducer types and electrode materials are investigated and compared. In addition, for edge-reflected SH0 resonators, stopband dispersion analysis is provided and the longitudinal ripples are suppressed. For edge-reflected SH0 resonators, a novel perfect edge reflector is proposed with elimination of longitudinal spurious ripples.

Extraction of frequency-dependent reflection, transmission, and scattering parameters for short metal reflectors from FEM-BEM simulations.

Reflectors comprised of only a single or a few electrodes provide controllable, weak reflectivity essential for surface acoustic wave (SAW) radi -frequency identification (RFID) tags. The reflection, transmission, and scattering parameters of such reflectors must be known as a function of frequency in order to be able to control the amplitudes of tag responses and to use phase-based encoding reliably. In this work, we present a method of extracting the main reflection, transmission, and scattering parameters for short metal reflectors as a function of frequency. We use test device S parameters obtained through finite- and boundaryelement method (FEM-BEM)-based simulations and, as an example, determine the reflection and transmission coefficients (their absolute values and phase angles) and the energy scattered into bulk for a few different single electrode reflectors. We compare these parameter values to earlier results. Although only used for simulated data in this work, the same method can be applied to measured data as well. Assuming the S parameters available, this method is very fast and does not require any heavy calculation or special software.

Z-path SAW RFID tag.

Surface acoustic wave (SAW) radio-frequency identification (RFID) tags are soon expected to be produced in very high volumes. The size and cost of a SAW RFID tag will be key parameters for many applications. Therefore, it is of primary importance to reduce the chip size. In this work, we describe the design principles of a 2.4-GHz SAW RFID tag that is significantly smaller than earlier reported tags. We also present simulated and experimental results. The coded signal should arrive at the reader with a certain delay (typically about 1 micros), i.e., after the reception of environmental echoes. If the tag uses a bidirectional interdigital transducer (IDT), space for the initial delay is needed on both sides of the IDT. In this work, we replace the bidirectional IDT by a unidirectional one. This halves the space required by the initial delay because all the code reflectors must now be placed on the same side of the IDT. We reduce tag size even further by using a Z-path geometry in which the same space in x-direction is used for both the initial delay and the code reflectors. Chip length is thus determined only by the space required by the code reflectors.

Narrow electrodes on YZ-LiNbOO3 as an alternative to etched grooves for dispersive delay lines.

Narrow, open-circuited aluminum electrodes can provide controllable, weak reflectivity necessary for many applications such as surface acoustic wave (SAW) tags and dispersive delay lines (DDLs). We show, using finiteand boundary element method (FEM-BEM) based simulations and experiments, that a reflectivity of 0.3% per wavelength can be achieved easily and controlled by varying the electrode width.

Hierarchical Cascading Algorithm for 2-D FEM Simulation of Finite SAW Devices.

Application of the finite-element method (FEM) for the simulation of surface acoustic wave (SAW) devices has been constrained by the large number of degrees of freedom required, resulting in large memory usage and long computation times. We propose a new 2-D algorithm that takes advantage of the periodic structure typical of SAW devices. The device is partitioned into small, repeatedly occurring building blocks. Only unique building blocks are simulated with FEM. The device geometry is presented as a hierarchical tree of cascading operations, where smaller blocks are combined into larger blocks. This is equivalent to the full FEM simulation of the device, implying the drastic reduction of memory consumption and simulation time for structures with a high degree of periodicity. The method is verified against FEM/BEM-based software. To ensure accurate and efficient simulation, the boundary conditions should be chosen according to the anisotropy of the substrate crystal.

SAW filters based on parallel-connected coupled resonator filters with offset frequencies.

The concept of coupled resonators is applied to synthesize surface acoustic wave filters. Employing two parallel-connected filter tracks, with a frequency shift imposed between them, a wide passband with low insertion loss together with well-controlled rejections is achieved. The operation of the two-track device is based on the mutual interaction of the individual transfer functions for the pair of tracks. Each track serves to contribute a part of the passband, enabling a wide band. Outside of the passband, the signals passing through the two channels may cancel each other, thus facilitating efficient control over the rejections. However, obtaining rejection stopbands at just the predetermined frequencies requires precise values for the materials parameters and a reliable fabrication process. Prototype devices fabricated with this approach are demonstrated both on quartz and, for the first time, on 42 degrees-LiTaO3. Results for two-track devices having either two or three transducers per track and operating either single-ended or with a balanced output are presented. The devices are designed employing the coupling-of-modes model and transmission-matrix approach, and the separate tracks are optimized simultaneously and independently. The center frequencies are 868 MHz and 1960 MHz. On quartz, a minimum insertion loss of 4 dB and a passband width of 0.23% are achieved at 868 MHz. On 42 degrees-LiTaO3, the corresponding figures of merit are 1.3 dB for minimum insertion loss and 4.1% bandwidth at 1960 MHz. The filters on 42 degrees-LiTaO3 also have remarkably flat passbands.

Double-resonance SAW filters.

A novel surface acoustic wave filter on a leaky-wave substrate is studied. It features a hiccup-type resonance occurring around a distributed gap between two long interdigital transducers. Compared to a classical coupled resonator filter, it enables a relatively narrow passband (1% to 2% of center frequency) with low insertion loss, steep skirts, and improved suppression levels. The structure consists of long transducers having the number of fingers greater than 1/K2 and 1/kappa where K2 is the coupling coefficient of the substrate material and kappa is the reflectivity per wavelength, separated with short transducer sections constituting a distributed gap. A strong, localized resonance is formed in the gap region, in addition to the resonance arising in the long structures-hence, the name double-resonance filter. The substrate studied here is 42 degrees-rotated lithium tantalite. We show experimental results for both single-ended and unbalanced-to-balanced filters at 1.6 GHz, having a minimum insertion loss of 1.07 dB, suppressions of 30 dB, and absolute -3-dB bandwidth of 29 MHz (1.9% of the center frequency). For the balanced device, the amplitude imbalance over the passband ranges from -0.6 dB to 2 dB and the phase imbalance from 1 degrees to 4.5 degrees. Furthermore, we have measured the acoustical power distributions using a scanning laser interferometer, and we compare these results with the profiles simulated using a coupling-of-modes model.

Side radiation of Rayleigh waves from synchronous SAW resonators.

Surface acoustic wave (SAW) resonators on lithium tantalate (LiTaO3) and lithium niobate (LiNbO3) are investigated. The amplitude of the acoustic fields in the resonators are measured using a scanning laser interferometer. The amplitude profiles of the surface vibrations reveal the presence of distinct acoustic beams radiated from the transducer region of the SAW resonators and propagating with low attenuation. We suggest that this radiation is generated by the charges accumulating at the tips of the finger electrodes. The periodic system of sources, namely oscillating charges at the fingertips, generates Rayleigh-wave beams in the perpendicular and oblique directions. Green's function theory is used to calculate the coupling strength and slowness of the Rayleigh waves on 42 degrees Y-cut LiTaO3 and Y-cut LiNbO3 substrates as a function of the propagation direction. Furthermore, the propagation angles of the Rayleigh-wave beams as a function of frequency are calculated. The computed angles are compared with the measured ones for both the LiTaO3 and LiNbO3 substrates.

Longitudinal leaky SAW resonators and filters on YZ-LiNbO3.

The high-phase velocity (above 6100 m/s in an aluminum (Al) grating on lithium niobate (LiNbOs)) of the longitudinal leaky surface acoustic wave (SAW) (LLSAW) mode makes it attractive for application in high-frequency SAW ladder filters in the 2-5 GHz range. We investigate the dependence of one-port synchronous LLSAW resonator performance on YZ-LiNbO3 on the metallization thickness and metallization ratio, both experimentally and theoretically. Our results indicate a strong dependence of the Q factor and resonance frequency on the aluminum thickness, with the optimal thickness that produces the highest Q values being about 8%. The optimal thickness increases with the metallization ratio. The observed behavior is interpreted with the help of simulations using a combined finite element method (FEM)/boundary element method (BEM) technique. As an application, bandpass filters have been fabricated in the 2.8 GHz frequency regime, based on LLSAWs. The synchronous resonators constituting the ladder filters operate in the fundamental mode. The filters feature low insertion losses below 3 dB and wide relative passbands of 4.5-5%.

Minimizing the bulk-wave scattering loss in dual-mode SAW devices.

The losses arising from the scattering of SAW into bulk waves in the nonsynchronous areas of SAW devices are studied numerically using the boundary element method combined with the finite element method. As a reference structure, we use a typical one-port hiccup resonator on 42 degrees Y-LiTaO3. Strong scattering into bulk wave occurs in the central gap due to an abrupt change in periodicity. To reduce the scattering, we replace the gap with electrodes having reduced pitches. We show that it is possible to significantly increase the Q-factor of the resonator while keeping the resonant frequency unchanged. Two types of structures are studied: the "distributed" gap and the "accordion" gap. To minimize the bulk-wave scattering in dual-mode SAW filters, we replace the metallized gaps in the traditional filter with distributed gaps. We find an optimal combination of pitch and metallization ratio in the gaps, reducing the insertion loss by 0.3 dB.

Extraction of the SAW attenuation parameter in periodic reflecting gratings.

In this paper, the extraction of the coupling-of-modes (COM) model attenuation parameter gamma in a finite grating is considered. We use test structures comprising identical transmitting and receiving transducers and a grating centered in the acoustic channel along the propagation direction of the surface acoustic wave (SAW). The extraction procedure proposed is based on studying the magnitude of the ratio of the reflection and transmission coefficients of the grating, R/T, obtained through time gating from the S parameter measurements of the test devices. In particular, we found that the level of the notches of R/T directly depends on the attenuation of SAW in the grating. A simple closed-form expression for the attenuation normalized to the grating length, gamma lambda0, depending on the characteristics of absolute value R/T, is given. The proposed method is applied to the measurement data for selected grating topologies to yield estimates of the attenuation.

Low-loss, multimode 5-IDT SAW filter.

Longitudinally coupled resonator filters provide unbalanced-balanced operation with wide bandwidth, low loss, and high suppression levels. However, reducing the insertion loss in the 1.8-2.2 GHz range remains a challenging problem because at high frequencies the resistive losses arising from the relatively wide aperture of the filter may degrade the performance. A 5-interdigital transducer (IDT) filter has six gaps at which the periodicity of the grating is broken, resulting in additional loss due to scattering into the bulk. In this paper, we show that replacing the gaps between the transducers with short transducer sections having their pitch different from that of the main transducers reduces the insertion loss of the device. We present devices with balun operation at 1842 MHz with wide bandwidth of 4.5% and -40 dB suppression, with a minimum insertion loss less than 1 dB in the best devices, and a maximum insertion loss of -1.2 dB in the passband. The passband is quite flat, with <1 dB ripple. We also discuss the layout of the contact pads and the connections, and its effect on the device performance and balance characteristics.

Two resonances of different nature in STW resonators with aperture- weighted metallization.

In a narrowband 2-port resonator on 37°-cut quartz with Al electrodes (h(Alu) = 100 nm) exploiting surface transverse waves (STW), we have observed parasitic ripples which have been attributed to transverse modes unusually situated on the left side of the main resonance. To suppress these modes, we have used metallization coefficient weighting across the aperture, with more metal in the middle and reduced metallization close to the busbars. The parasitic modes indeed disappeared, but at significantly higher frequency, we have found an additional strong response which does not exist in a resonator with uniform electrode metallization. 3-D simulations showed that the structure has another very different mode, with the wave propagating mainly along the edge of the busbars, but excited with the interdigital electrode system.

Short reflectors operating at the fundamental and second harmonics on 128 degree LiNbO3.

In this work, we study numerically the operation of surface acoustic wave (SAW) reflectors comprising a small number of electrodes on the 128 degree YX-cut lithium niobate (LiNbO3) substrate. The electrodes have a finite thickness, and they are either open circuited or grounded. The center-to-center distance between adjacent electrodes d corresponds roughly either to half of the characteristic wavelength d proportional to lambda0/2 or to d proportional to lambda0, for the reflectors operating at the fundamental and second harmonic modes, respectively. We use software based on the finite-element and boundary-element methods (FEM/BEM) for numerical experiments with a tailored test structure having 3 interdigital transducers (IDTs), simulating experimental conditions with an incident wave and reflected and transmitted SAWs. Using the fast Fourier transform (FFT) and time-gating techniques, calculation of the Y-parameters in a wide frequency range with rather a small step allows us to determine the reflection coefficients, and to estimate the energy loss due to bulk-wave scattering. The detailed dependences of the attenuation and reflectivity on the metallization ratio and the electrode thickness are given for the classic 128 degree-cut of LiNbO3.

Second-harmonic reflectors on 128 degrees LiNbO3.

In this work, we study theoretically the operation of long surface acoustic wave reflectors, comprising a large number of electrodes, at the fundamental and second harmonic frequencies on the 128 degrees LiNbO3 substrate for various electrode thicknesses and metallization ratios. Numerical simulations utilizing tailored test structures and time gating indicate that the reflectivity of the second-harmonic reflectors can be very high for certain geometries. Furthermore, our simulations suggest that inside the stopband the total losses for the second harmonic are of the same order as those for operation at the fundamental harmonic.

Unidirectional SAW transducer for gigahertz frequencies.

A single-phase unidirectional transducer (SPUDT) structure using lambda/4 and wider electrodes is introduced. The considerable difference between the reflectivity of short-circuited lambda/4 electrodes and that of floating lambda/2-wide electrodes on 128 degree lithium niobate (LiNbO3) is exploited. The surface acoustic wave (SAW) device operating at 2.45 GHz has critical dimensions of about 0.4 microm, accessible for standard optical lithography.

Phases of the SAW reflection and transmission coefficients for short reflectors on 128 degree LiNbO3.

We study numerically the phase of surface acoustic waves reflected by or transmitted through short reflectors comprising only 1-3 aluminium electrodes on 128 degree YX-cut lithium niobate (LiNbO3). The electrodes have a finite thickness, and they are either open-circuited or grounded. The center-to-center distance between adjacent electrodes d corresponds roughly either to half of the characteristic wavelength d proportional to lambda0/2 or to d proportional to lambda0, for the reflectors operating at the fundamental and second harmonic modes, respectively. We use software based on the finite-element and boundary-element methods (FEM/BEM) for numerical experiments with a tailored test structure having 3 interdigital transducers (IDTs), simulating experimental conditions with an incident wave and reflected and transmitted surface acoustic wave (SAW). Using artificial enhancement of time resolution in conjunction with the fast Fourier transform (FFT) and time-gating, calculation of the Y-parameters in a relatively wide frequency range allows us to determine the phase of the reflection and transmission coefficients.

SPUDT filters for the 2.45 GHz ISM band.

Filters based on using single-phase, unidirectional transducers (SPUDT) consisting of lambda/4 and wider electrodes are presented. The design variants exploit the significant difference between the reflectivity of short-circuited lambda/4 electrodes and that of floating wide electrodes on 128 degree lithium niobate (LiNbO3). The surface acoustic wave (SAW) devices operating at 2.45 GHz have critical dimensions of about 0.3-0.4 microm, accessible to standard optical lithography. When matched, the fabricated SPUDT filters exhibit minimum insertion losses of 5.5-7.9 dB together with 3 dB passbands of 89-102 MHz. The majority of the insertion loss can be attributed to the attenuation on free surface and inside the grating, and to the resistivity of the electrodes.

SAW tags for the 6-GHz range.

The possibility of using 6-GHz frequencies for passive SAW tags is discussed. An example of an inline 6-GHz SAW tag design is presented. Supposing that an ultrawide frequency bandwidth B = 775 MHz can be used, the tag dimensions can be significantly reduced and the loss of reflected response remains at an acceptable level of around 50 dB. The devices were manufactured using E-beam lithography and, probed on wafer, show performance close to predicted, including the loss level of approximately 55 dB. The possibility of high-throughput manufacturing by combining nano-imprint and e-beam lithography is discussed.

Error probability for RFID SAW tags with pulse position coding and peak-pulse detection.

This paper addresses the code reading error probability (EP) in radio-frequency identification (RFID) SAW tags with pulse position coding (PPC) and peak-pulse detection. EP is found in a most general form, assuming M groups of codes with N slots each and allowing individual SNRs in each slot. The basic case of zero signal in all off-pulses and equal signals in all on-pulses is investigated in detail. We show that if a SAW-tag with PPC is designed such that the spurious responses are attenuated by more than 20 dB below on-pulses, then EP can be achieved at the level of 10(-8) (one false read per 108 readings) with SNR >17 dB for any reasonable M and N. The tag reader range is estimated as a function of the transmitted power and EP.